The health care bill that Senate Republicans introduced today reflects just one of their responsibilities. A more basic job is simply finishing the federal budget. The United States will spend around $4 trillion next year funding everything from Medicare to food stamps to weapons to highways. It's sometimes a dramatic battle over that budget that can even include a government shutdown from time to time. Yet, things don't actually change from year to year all that much. Here's Stacey Vanek Smith of our Planet Money podcast.

STACEY VANEK SMITH, BYLINE: We spend $1.5 billion every day on defense. It accounts for 15 percent of the federal budget. One of the biggest changes President Trump proposed for next year's budget is an increase in defense spending. And that money pays for everything from airplanes to military bases to toilet paper. And most of it goes to the Defense Department. A big chunk also goes to the Energy Department. That's where Bill Valdez worked for more than 20 years.

BILL VALDEZ: The nation's nuclear stockpiles are developed and maintained by the Department of Energy.

VANEK SMITH: Oh, wow. So you know about the nukes?

VALDEZ: Yep.

VANEK SMITH: The Energy Department also does climate research and environmental cleanup. Valdez worked for many different parts of the Energy Department and wrote a lot of budgets. He says, defense spending has always been a political battleground, not that you'd know it. The percentage of the U.S. budget that goes to defense hasn't really changed in the last 20 years, and that is actually true for most of the federal budget - Medicaid, Veterans Affairs, education. And Valdez says the reason for this is kind of surprising.

VALDEZ: You know, the real thing that keeps the budget stable is that there are all these special interest groups.

VANEK SMITH: Lobbyists, hundreds of them, making deals with different states and politicians, showing up at government agencies.

VALDEZ: Special interest groups are always coming to the Department of Energy, to all the agencies, saying, these are our priorities, and we hope they're reflected in the budget.

VANEK SMITH: Valdez says the status quo is really a hard-earned equilibrium. And it's a really tough balance for any new administration to try and disrupt. And Valdez says this is a good thing in a lot of ways. It creates stability and means things don't change too fast.

So we actually have lobbyists to thank for our stable budget?

VALDEZ: Yeah. In many respects, yeah.

VANEK SMITH: It's like a little shoutout to the lobbyists.

VALDEZ: (Laughter) Well, I mean...

VANEK SMITH: They get so little love. I feel like...

VALDEZ: (Laughter).

VANEK SMITH: ...That's OK, right? Don't you think that maybe the budget should be their valentine?

VALDEZ: (Laughter) Well, it often is.

VANEK SMITH: To pay for the increase in defense spending, Trump's budget proposed big cuts in other parts of the budget, like education and health care. Valdez says Trump will have to get through the lobbyists for those programs first.